The present invention pertains to a configuration as well as an operating method of a Passive Optical Network system in which a plurality of subscriber connection devices share a transmission line.
In order to transmit and receive high-capacity image signals or data via a communication network, the attainment of higher speeds and larger bandwidths is being advanced as well in the access network connecting the subscriber to the communication network, the introduction of a Passive Optical Network system (below called “PON”) defined in Recommendation G984.1-3 et cetera of the International Telecommunications Union (below called “ITU-T”) being aimed for. A PON is a system connecting an Optical Line Termination (below called “OLT”) connected to a host communication network; and Optical Network Units (below called “ONU”) accommodating a plurality of subscriber terminals (PCs or telephones) connected with a passive optical network consisting of a trunk optical fiber and branch optical fibers. Specifically, it is a system carrying out communication with a mode in which the signals coming from the terminals (PCs and the like) connected to each ONU are sent as optical signals from branch optical fibers via an optical splitter and optically multiplexed (by time division) with the trunk optical fiber to the OLT and the OLT carries out communication processing of the signals from each ONU and either transmits them to the host communication network or transmits them to another ONU connected to the OLT.
Each ONU can, e.g. as defined in Chs. 8 and 9 of Recommendation G984.1, have the combined fiber length (transmission distance) of the trunk optical fiber and the branch optical fiber from the OLT set arbitrarily within 0-20 km, 20-40 km, and 40-60 km ranges. That is to say that since there is a random variation in the transmission delay between each ONU and the OLT, there is a possibility, even if each ONU transmits a signal, that the optical signals output from each ONU on the trunk optical fiber collide and interfere with each other. For this reason, by using e.g. ranging technology defined in Ch. 10 of Recommendation G984.3, the delay in the output signal of each ONU is regulated, after carrying out a distance measurement between the OLT and the ONU, as if each ONU had been set to an equal distance (e.g. 20 km) from the OLT. Then, when the OLT decides, for each ONU, on the signal bandwidth in which transmission is granted, it assigns transmission timing to each ONU so that the optical signals from each ONU on the trunk optical fiber do not collide or interfere, assuming that the distance to each OLT is an equal distance. Further, at the head of the signal from each ONU, if what is concerned is e.g. that specified in Section 8.3.3 of Recommendation G.984.2, there are added a guard time for interference prevention consisting of at most 12 bytes, a preamble utilized for the determination of a signal discrimination threshold value of the internal OLT receiver and clock extraction, and a delimiter discriminating breaks in the received signals so as to make it possible to discriminate and process the signals from each ONU multiplexed by the OLT on the trunk optical fiber.
However, as mentioned above, even if collisions between the optical signals are avoided, the received levels of the optical signals at the OLT from each ONU differ greatly since the actual lengths of the optical fibers differ. For this reason, it is a configuration in which the preamble of the optical signal is used, the received level of the optical signal is measured, the signal from each ONU is received after adjusting the signal discrimination threshold value and the clock phase of the receiver inside the OLT to correspond to the concerned ONU each time a signal from each ONU is received at the OLT. Further, as disclosed in JP-A-2002-57627, it is possible, instead of adjusting the signal discrimination threshold value, to choose a configuration in which the signal, after measurement of the received signal level, is received after amplifying the received signal to a prescribed level using an optical amplifier.
The development and introduction of the PON started from handling signals with low speeds such as 64 kbit/s and is now proceeding with the introduction of the higher-speed BPON (Broadband PON) and GPON (Gigabit PON) handling signals on the order of 2.4 Gbit/s. Moreover, in the future, there is demanded the implementation of high-speed PONs capable of handling signals from 10 Gbit/s to 40 Gbit/s. Also, these PONs have gradually come from handling conventional fixed-length signals to burst-shaped variable-length signals (burst signals).
As mentioned above, since the received levels of the optical signals differ greatly, it is demanded of the OLT burst signal receiver circuit receiving multiplexed optical signals from each ONU both a large dynamic range capable of handling this random variation and a high-speed tracking ability devised to be able to accurately receive optical signals from each ONU in a short time. However, if e.g. the transmission speed of the optical signals is 1.2 Gbit/s, in order to attain a high-speed tracking ability performing signal discrimination threshold value determination of received signals and clock adjustment in a short time in the OLT receiver circuit, it is demanded to implement a receiver circuit using high-speed devices operating at speeds (on the order of 4 Gbit/s to 10 Gbit/s) at least several times higher than the optical signal transmission speed. This trend will be the same for further speed increases in the future. An economical supply of receiver circuits using devices able to handle these kinds of high transmission speeds is not straightforward. Of course, if a configuration is adopted in which, without speeding up the receiver circuit, the preamble preceding the burst signal is made longer and the signals from each ONU are tracked slowly, the tracking ability performance demanded of the receiver circuit is relaxed. However, if the preamble is made longer, the net bandwidth that can be used for the transmission of the signals from each ONU decreases and the bandwidth utilization efficiency drops. That is to say that it ends up going against the objective of speeding up the system. Also, in a configuration in which an amplifier such as in JP-A-2002-57627 is introduced, the configuration of the receiver circuit becomes a little easier, but there is no change in the need for receiver level measurements of the received optical signals, and since the configuration ends up becoming one using a costly amplifier using active components, it becomes difficult to attain the goal of making PONs economical.